So far, as a substrate of a flat display such as a liquid crystal display and an EL display, a glass substrate has been widely used.
In particular, an electronic device such as a thin film transistor type active matrix liquid crystal display (TFT-LCD), being thin in thickness and less in the power consumption, has been used in various applications such as car navigation, a finder of a digital camera, a monitor for a personal computer and a TV.
In order to drive a liquid crystal display, it is necessary to form a driving element typified by a TFT element on a glass substrate. In a process of producing a TFT element, a transparent conductive film, an insulating film, a semiconductor film, a metal film and the like are deposited on a glass substrate. Furthermore, in a photolithography etching process, the glass substrate is subjected to various kinds of heat treatments and chemical treatments. For instance, in a TFT type active matrix liquid crystal display, an insulating film and a transparent conductive film are deposited on a glass substrate. Furthermore, a lot of TFTs (thin film transistor) made of amorphous silicon or polycrystalline silicon are formed on the glass substrate by a photolithography-etching process. In such producing processes, the glass substrate is subjected to a heat treatment at a temperature in the range of 300 to 600° C. as well as a treatment of various chemicals such as hydrochloric acid, sulfuric acid, an alkali solution, hydrofluoric acid and buffered hydrofluoric acid. Accordingly, on a glass substrate for use in a TFT liquid crystal display, following characteristics are demanded.
(1) When an alkali metal oxide is contained in glass, an alkali ion diffuses in a deposited semiconductor material during the heat treatment to deteriorate the characteristics of a film; accordingly, an alkali metal oxide is substantially not contained.
(2) The resistance to a solution of acid, alkali and the like used in the photolithography-etching process, that is, the chemical resistance is excellent.
(3) In the steps of deposition, annealing and the like, the glass substrate is exposed to a high temperature. At that time, the thermal shrinkage of the glass substrate is desired small. That is, this is because, when the thermal shrinkage is large, a pattern deviation of a circuit formed on the substrate is caused. From the viewpoint of making the thermal shrinkage smaller, the strain point of glass is advantageously higher.
Furthermore, for a glass substrate for a TFT liquid crystal display, the following characteristics are demanded other than the above.
(4) From the viewpoint of generating no foreign matter in the glass in the steps of melting or forming glass, the devitrification resistance is necessary to be excellent. In particular, when glass is formed in accordance with a down draw method such as an overflow down draw method, the devitrification resistance of glass is important. When a glass forming temperature is taken into consideration, a liquidus temperature is demanded to be 1200° C. or less.
(5) The density is desired low to make a liquid crystal display lighter in weight. In particular, a glass substrate that is mounted on a note-type personal computer is strongly demanded to be light in weight, specifically, to be 2.50 g/cm3 or less.
(6) The flatness of a surface is necessarily high. For instance, in a liquid crystal display, a liquid crystal layer interposed between two thin glass substrates works as an optical shutter and the layer shields or transmits light to execute display. The liquid crystal layer is maintained to have a very thin thickness such as several to several tens micrometers. Accordingly, the flatness of a surface of the glass substrate, in particular, the unevenness of micrometer level called as undulation tends to affect on a thickness of the liquid crystal layer (called as a cell gap). That is, when the undulation of a surface is large, the display defect such as the display unevenness is caused.
(7) The undulation of a glass substrate is necessarily small. Recently, in a liquid crystal display, for the purpose of high-speed response and high precision, a cell gap tends to be smaller and smaller; accordingly, it becomes more and more important to reduce the undulation of a surface of the glass substrate used herein. A most effective method for reducing the undulation of a surface of the glass substrate is to precisely polish a surface of a formed glass substrate. However, the method is very high in the production cost of the glass substrate. Accordingly, at present, a forming method such as an overflow down draw method or a float method is used for forming a glass substrate as small as possible in the undulation of a surface, and they are then shipped without polishing or after being slightly polished (touch polishing).
In order to satisfy these characteristics, various kinds of glass substrates have been proposed (such as patent literature 1).    Patent literature 1: JP-A-08-811920